Error reflection for Fin

Add reflection-based error message improvements for Fin literals. Now,
instead of a collection of internal implementation details, a friendly
and descriptive message is returned.

See test/error005 for a demo.
Cabal file boilerplate for new tests and source
Merge pull request #1803 from david-christiansen/fin-error-reflection

Error reflection for Fin
Lambdas match on pattern expressions

i.e. inPattern should be true so that lambdas can have @-patterns, in
particular.
Merge branch 'master' of github.com:idris-lang/Idris-dev
Add missing pattern in warnDisamb